src: Use bytes.Equal instead of bytes.Compare where possible.

bytes.Equal is simpler to read and should also be faster because
of short-circuiting and assembly implementations.

Change generated automatically using:
  gofmt -r 'bytes.Compare(a, b) == 0 -> bytes.Equal(a, b)'
  gofmt -r 'bytes.Compare(a, b) != 0 -> !bytes.Equal(a, b)'

R=golang-dev, dave, adg, rsc
CC=golang-dev
https://codereview.appspot.com/7038051

Committer: Andrew Gerrand <adg@golang.org>